1. Field of the Invention
The present invention generally relates to signal transmission and receiving means for use in a wireless cable pay television system and, more particularly to a system which conveys addressing data to one-way addressable set top converters, scrambles/unscrambles video signals, masks/unmasks audio signals and provides additional data paths per channel. Wireless cable systems provide pay television services by broadcasting television signals over the air using microwave transmitters, whereas classical cable TV systems use coax cable to distribute signals.
2. Description of the Prior Art
Cable television has become very popular in the United States, and the operation of cable systems has become quite profitable. These systems use coaxial cable and amplifiers to distribute television signals, data and, in some cases, FM radio signals to subscribers. Through the years, equipments used in cable television for generating, receiving and processing signals have become very sophisticated and cost effective. Modern systems provide some or all of the following capabilities at low cost:
1. Sophisticated baseband or RF video scrambling. Several modes of scrambling are often provided.
2. Audio masking or scrambling.
3. One or two-way addressable converters, with data rates up to 15 kbits/second.
4. Many subscriber features, such as remote control, parental control and delivery of programs on a pay per view basis.
In recent years, an alternative means for delivering "cable TV" programming to subscribers has become available through actions taken by the Federal Communications Commission (FCC). Up to thirty-three channels are now available for over-the-air transmission of television signals in the frequency ranges of 2,150 to 2,162 MHz, and 2,500 to 2,690 MHz. These frequencies are licensed by the Multi-point Distribution Service (MDS), Instructional Television Fixed Service (ITFS), Multi-point, Multi-channel Distribution Service (MMDS) and Operational Fixed Service (OFS). FIG. 1 illustrates the microwave frequencies available from these FCC bureaus.
The delivery of pay TV programming using microwave transmission has become associated with the name "Wireless Cable", since "cable" programming is delivered using a "wireless" transmission method. Equipments used in wireless cable systems must perform most, if not all, of the functions performed by cable equipment. However, wireless cable systems impose technical problems that do not exist for cable systems. Thus, video scrambling methods and data transmission methods which are more than adequate for cable systems may be totally unacceptable in a wireless cable environment.
The basic elements of a wireless cable system are illustrated in FIG. 2. At the transmitter site 10, video/audio signals (from satellite, tape or other source) are applied to a modulator 12 and then to a microwave transmitter 14. Data from an addressing computer 16 is applied to a modulator 13 and then to a microwave transmitter 15. The outputs of the transmitters 14 and 15 are combined in a channel combiner 18 and applied to a transmitting antenna 19 using waveguide.
Per current FCC regulations, signals are broadcast to receive sites at power levels of 10, 50 or 100 watts per channel. The output signals from modulators 12 and 13 are conveyed to converters (not shown) so that the modulated signals are within the spectrum allocated for the channel. Additionally, if scrambling is performed, it is assumed that the modulator also performs all scrambling functions.
Each receive site 20 consists of a receiving antenna 21, block down converter 22, set top converter 23, and TV receiver 24. The block down converter 22 is mounted close to the receiving antenna 21 and converts all of the microwave channels to the VHF frequency band. At these lower frequencies, the channels can be cabled into the home using standard coax with little signal loss. Channels are selected using the set top converter 23. The converter 23 typically provides a channel 2, 3 or 4 output to the TV receiver 24. If one or more channels are scrambled, the converter contains the appropriate descrambling circuitry.
A wireless cable system is different from a typical cable system in many respects. Some of these differences present significant technical challenges to designers of wireless cable systems, particularly in the areas of scrambling and data delivery. Some specific problems which are unique to wireless cable systems are described below:
1. Transmitter Non-Linearity: In cable systems, the output of the modulator for each channel is applied to an up converter. The up converter's output is at a low power level, and therefore the up converter can be very linear. In wireless cable systems, the transmitter must provide 10, 50 or 100 watts. As a result, even the best transmitters available today are not as linear as cable up converters. This transmitter non-linearity can cause problems when typical sync suppression or sync offset scrambling systems are used. Specifically, in any video scrambling system which attenuates or offsets selected sync pulses, while keeping the unselected pulses unaltered, transmitter non-linearity usually results in streaking in the descrambled picture. Streaking occurs because the attenuated (or offset) sync pulses require less power output from the transmitter as compared to a standard sync pulse. The non-linear characteristic of the transmitter results in a "crushing", or reduction, of unsuppressed sync pulses as compared to the suppressed sync pulses. In the descrambled signal, the amplitude of the sync pulses transmitted in a suppressed, or offset manner is different from those pulses sent normally. Since TV receivers typically use sync amplitude to establish black level, a streaking phenomenon occurs due the variation in sync pulse amplitude.
2. Transmitter Automatic Level Control (ALC): Modern microwave transmitters contain circuitry which automatically controls the power output level. These circuits typically use a peak detector to sense the maximum level of the incoming video intermediate frequency (IF) signal. Transmitter output is set to the desired level (10, 50 or 100 watts) corresponding to the peak video IF level sensed. A similar requirement does not exist for cable systems. Some scrambling systems suitable for cable are not compatible with microwave transmitter ALC circuit operation.
3. Multi-path: Multi-path conditions are virtually non-existent in cable systems. In wireless cable, multi-path conditions can occur frequently. Multi-path conditions can be very disruptive to amplitude modulated signals. Since the picture carrier is amplitude modulated, multipath produces ghosts in the received picture. Any data information conveyed using amplitude modulation (such as data sent in the vertical blanking interval) is very susceptible to corruption by multi-path conditions.
4. Signal Strength Variations: In cable systems, the level of signals applied to the set top converter can usually be confined within a 10 dB range, and any variations which may occur do so very slowly. In wireless cable, signal levels can vary over a range of 30 dB or more, and the rate of change can be rapid (300 Hz or greater).
5. Available Spectrum for Data Carriers: In cable systems, that portion of the frequency spectrum just above 108 MHz (which is the top of the FM radio band) is often used for dedicated data carriers. In wireless cable, the FCC has allocated a limited number of channels, and no "spare" spectrum exists for dedicated data carriers. Furthermore, since channel capacity is limited to a maximum of thirty-three channels, it would be too wasteful to dedicate one channel simply for data transmission.
Methods employed for data delivery and scrambling in cable systems are, in many cases, inadequate for use in wireless cable. A need exists for a low-cost transmission/receiving system for wireless cable which can convey data to addressable set-top converters, scramble/descramble video, mask/unmask audio, and be impervious to transmission problems that are unique to wireless cable systems.